The present invention relates to a polarization independent-type semiconductor optical amplifier, more specifically to a polarization independent-type optical amplifier for use in wavelength division multiplexing (WDM) communication systems, which has large fiber out saturation powers in a small size and at low consumed electric power.
Recently, corresponding to the drastic increase of communication demand, wavelength division multiplexing systems for multiplexing signal light of a plurality of different wavelengths and transmitting the signal light concurrently in one optical fiber are progressively developed. Such wavelength division multiplexing system includes a number of optical parts for combining, switching and dividing optical signals, and the optical signals are attenuated due to losses in the respective optical parts.
Optical amplifiers are used for compensating such attenuation. A very larger number of optical amplifiers are required in comparison with a number of optical amplifiers used in the conventional optical fiber systems. It is required that the optical amplifiers are small sized and operable at low power consumption.
In addition, it is required that such optical amplifiers used in in-line have less polarization dependence of gains because polarization states of input signal lights are random, and have large fiber out saturation powers so as to have wide input dynamic ranges because fluctuations of power levels of input signal light are large.
Among such various optical amplifiers, semiconductor optical amplifiers (SOAs) are small sized and have small power consumption, and can be designed to be polarization-independent. The semiconductor optical amplifiers are expected to be a loss compensating optical amplifier suitably used in the wavelength division multiplexing systems.
Such polarization independent-type semiconductor optical amplifiers for a 1.55 xcexcm-band in the wavelength band used in the fiber optical communication have been conventionally developed. Such polarization independent-type semiconductor optical amplifier will be explained below.
An internal gain is a gain of the optical amplifier itself. A fiber to fiber gain is a gain of a system as a whole including an optical amplifier provided between an optical fiber on the input side and an optical fiber on the output side with an optical system for optical coupling, such as a lens, etc., disposed therebetween, which takes into account a loss of the system between the exit end face of the input side optical fiber and the incidence end face of the output side optical fiber.
A chip out saturation power is a chip out power given when an internal gain is decreased by 3 dB. A fiber out saturation power is a fiber output power given when a fiber to fiber gain is decreased by 3 dB.
As a semiconductor optical amplifier using a strain-free bulk active layer, P. Doussiere et al., Alcatel have realized a device which includes a bulk active layer of a 430 nm-thick and a 500 nm-width, and which has, at 800 xcexcm-device length and at 200 mA injection current, a below 0.5 dB inter-polarization gain difference, a 29 dB fiber to fiber gain and a +9.0 dBm fiber out saturation power (see, e.g., P. Doussiere et al., IEEE Photon. Technol. Lett., vol. 6, pp. 170-172, 1994 and P. Doussiere et al, OAA ""95, pp. 119-122).
As a semiconductor optical amplifier using a strained Multiple Quantum Well (MQW) active layer formed of a strain-free well layer and an tensile strained barrier layer, Magari et al., NTT realized a device including a strained MQW layer which is formed of 10 well layers each having a 5 nm-thick and a 0% strain amount and 11 barrier layers each having a 5 nm-thick and a xe2x88x921.7% strain amount, and is sandwiched between 50 nm-thick and 100 nm-thick separate confinement heterostructure (SCH) layers, and which has, at a 660 xcexcm-device length and 200 mA injection current, a below 1.0 dB inter-polarization gain difference, a 27 dB internal gain (a 13 dB fiber to fiber gain), a +14.0 dBm chip out saturation power (a +7.0 dBm fiber out saturation power) (see, e.g., K. Magari et al., IEEE Photon. Technol. Lett., vol. 2, pp. 556-558, 1990, K. Magari et al., IEEE Photon. Technol. Lett., vol. 3, pp. 998-1000, 1991, and K. Magari et al., IEEE J. Quantum Electron., vol. 30, pp. 695-702, 1994).
As a semiconductor optical amplifier similarly using a strained MQW active layer formed of a strain-free well layer and a tensile strained barrier layer, A. E. Kelly, et al., BT realized a device including a strained MQW layer which is formed of 10 well layers of a 0% strain amount and 11 barrier layers of a xe2x88x920.67% strain amount and is sandwiched between 25 nm-thick SCH layers, and which has, at a 2000 xcexcm-device length and 200 mA injection current, a below 0.5 dB inter-polarization gain difference, a 27 dB fiber to fiber gain and a +7.5 dBm fiber out saturation power (see, e.g., A. E. Kelly et al., Electron Lett., vol. 32, pp. 1835-1836, 1996 and A. E. Kelly et al., Electron Lett., vol. 33, pp. 536-538, 1997).
As a semiconductor optical amplifier using a strained MQW active layer formed of a compressive strained quantum well layer, a tensile strained quantum well layer and a strain-free barrier layer, M. A. Newkirk et al., ATT realized a device including a strained MQW layer which is formed of 3 compressive strained quantum well layers each having a 3.5 nm-thick and a +1.0% strain amount, 3 tensile strained quantum well layers each having a 16.0 nm-thick and a xe2x88x921.0% strain amount, and 7 barrier layers each having a 10 nm-thick and a 0% strain amount, and which has, at a 625 xcexcm-device length and 150 mA injection current, a below 1.0 dB inter-polarization gain difference, a 13 dB internal gain (a 4.4 dB fiber to fiber gain) and a +11.1 dBm chip out saturation power (a +6.8 dBm fiber out saturation power) (see, e.g., M. A. Newkirk et al., IEEE Photon. Technol. Lett., vol. 4, pp, 406-408, 1993).
As a semiconductor optical amplifier using a strained MQW active layer formed of a compressive strained quantum well layer and a tensile strained barrier layer, D. Sigogne et al., CNET realized a device which includes a strained MQW layer formed of 16 compressive strained quantum well layers each having a 8 nm-thick and a +1.1% strain amount and 16 tensile strained barrier layers of a 7 nm-thick and a xe2x88x920.9% strain amount, and which has, at a 940 xcexcm-device length and 150 mA injection current, a below 1.0 dB inter-polarization gain difference, a 23 dB fiber to fiber gain and a +7.0 dBm chip out saturation power (a +3.5 dBm fiber out saturation power) (see, e.g., A. Ougazzaden et al., Electron. Lett., vol. 31, pp. 1242-1244, 1955, D. Sigogne et al., ECOC95, pp. 267-270, and D. Sigogne et al., Electron. Lett., vol. 32, pp. 1403-1405, 1996).
As a semiconductor optical amplifier using a tensile strained bulk active layer, J. Y. Emery et al., Alcatel realized a device which includes a 200 nm-thick bulk active layer sandwiched on both sides thereof by 100 nm-separate confinement heterostructure layers and having a xe2x88x920.15% tensile strain at a 1.2 xcexcm-active layer width, and which has, at a 1000 xcexcm-device length and 200 mA injection current, a below 0.3 dB inter-polarization gain difference, a 29 dB fiber to fiber gain and a +9.5 dBm fiber out saturation power (see, e.g., J. Y. Emery et al., ECCO96, vol. 3, pp. 165-168 and J. Y. Emery et al., Electron. Lett., vol. 33, pp. 1083-1084, 1997).
Polarization independent-type semiconductor optical amplifiers of various active layer structures as described above have been studied. In such semiconductor optical amplifiers, in order to obtain a wide dynamic range it is required that a fiber out saturation power, which provides an upper limit of the dynamic range, is as large as possible. For example, for a semiconductor optical amplifier for 1.55 xcexcm-band having polarization dependency, the MQW active layer structure can provide a +19.5 dBm chip out saturation power.
However, in the polarization independent-type semiconductor optical amplifier for 1.55 xcexcm-band, even a maximum fiber out saturation power cannot exceed +9.5 dBm obtained by using the tensile strained bulk active layer of the above-described J. Y. Emery et al., Alcatel. The optical amplifier is inferior to the polarization dependent-type optical amplifier by even 7.5 dB, taking into account a 2.5 dB fiber coupling loss.
A cause for such low fiber out saturation powers of the polarization independent-type semiconductor optical amplifiers is that structural restrictions of the active layers imposed for the optical amplifiers to be polarization independent hinders high out saturation power.
In the case that the strain-free bulk active layer is used as in P. Doussiere et al., Alcatel, the active layer has rectangular section for making the optical confinement in the active layer polarization-independent. The lower limit of dimensions of the section of the active layer is 300 nm-square due to fabrication limitations. The upper limit is restricted to 600 nm-square due to conditions for maintaining fundamental modes. Freedom of design of the device is low.
In the case that the strained MQW layer is used, in order to nullify increased material gain for TE polarization light in the active layer due to the quantum effect and increased optical confinement for TE polarization light in the flat active layer, a large tensile strain must be used for increased material gain for TM polarization light.
However, in the case that such large tensile strain is used, because a gain peak wavelength is made shorter by both quantum effect and tensile strain effect, and also by band filling effect due to injection current increase, in order to obtain a necessary gain near a 1.55 xcexcm-wavelength, a restriction that tensile strain is applied to the barrier layer, or the quantum well layer is made thick to depress the shortening of the wavelength due to the quantum effect is applied. Accordingly, a problem is that freedom of structural design for obtaining large fiber out saturation powers is much decreased.
Here, with reference to FIG. 14, the conventional polarization independent-type semiconductor optical amplifier using the tensile strained bulk active layer of Alcatel will be explained.
FIG. 14 is a diagrammatic perspective view of the conventional polarization independent-type semiconductor optical amplifier. In the forward half portion of the semiconductor optical amplifier, a p-type InP buried layer 37, proton-injection regions 38, 39, a p-type InGaAs contact layer 40 and a p-side electrode 42 are not shown so as to show the structure of the active layer. A spot size conversion region, a window region, etc., are left out of FIG. 14.
In this polarization independent-type semiconductor optical amplifier, 100 nm-thick InGaAsP separate confinement heterostructure layers (SCH layers) 33, 35 are provided on and the underside of a 200 nm-thick InGaAsP strained bulk active layer 34, and a stripe width is 1.2 xcexcm.
The optical axis of the striped InGaAsP strained bulk active layer 34 intersects at a 7xc2x0 inclination angle, a normal of the light input/output end faces.
This optical amplifier is so designed that the active layer which is as thick as 200 nm is used to make an optical confinement ratio between TE and TM polarization lights (TE/TM) small, whereby a required tensile strain is depressed to be as low as xe2x88x920.15%. Resultantly, an inter-polarization gain difference can be small, whereby a signal input light 46 entering at one end face is amplified independent of polarization to be outputted as amplified output light 47.
Antireflection coating films (AR films) 44, 45 are provided on both end faces for suppressing resonance of the signal input light 46.
However, as described above, the polarization independent-type semiconductor optical amplifier using this tensile strained bulk active layer has a +9.5 dBm fiber out saturation power. This fiber out saturation power is still considerable low, even taking into account fiber coupling loss of 2.5 dB, in comparison with a +19.5 dBm chip out saturation power of a polarization dependent-type semiconductor optical amplifier.
In this polarization independent-type semiconductor optical amplifier using tensile strained bulk active layer, the tensile strained bulk active layer still has not a critical film thickness for strain relaxation, and accordingly a larger strain can be applied. Accordingly, the active layer structure is changed to thereby increase the chip out saturation power, whereby the fiber out saturation power can be increased.
An object of the present invention is to obtain increased fiber out saturation power without lessening structure design freedom, by making no inter-polarization gain difference while increasing output saturation power.
Means for achieving the object of the present invention will be explained with reference to FIGS. 1A and 1B. FIG. 1A is an upper side view, and FIG. 1B is a diagrammatic sectional view which is normal to the optical axis. In the drawings, reference numbers 1 and 6 represent a clad layer, reference number 4 indicates a striped mesa, and reference number 5 denotes a buried layer.
The polarization independent-type semiconductor optical amplifier according to the present invention is characterized in that the polarization independent-type semiconductor optical amplifier comprises: a strained bulk active layer 3 of a bulk crystal having a tensile strain introduced into, resonance of light due to reflection between a light incident end face 7 and a light exit end face 8 being depressed, an optical signal 9 being incident on the light incident end face 7, current being injected into the strained bulk active layer 3 to amplify the optical signal 9 by a stimulated emission effect, the amplified signal 10 exiting at the light exit end face 8, and a signal transmission gain of the amplified signal 10 being substantially constant, independent of a polarized state of the incident optical signal 9, in which a thickness of the strained bulk active layer 3 is 20 nmxcx9c200 nm, and a strain amount is xe2x88x920.09% xcx9cxe2x88x920.60%.
Thus, the strained bulk active layer 3 forming the polarization independent-type semiconductor optical amplifier has a thickness d of 20 nmxcx9c100 nm and a strain amount of xe2x88x920.09%xcx9cxe2x88x920.60%, whereby large output saturation power can be provided while polarization independence being retained.
That is, generally an output saturation power Ps of a semiconductor optical amplifier is expressed by
Ps=(wxc3x97d/xcex93)xc3x97hv/(xcfx84xc3x97a)
where a thickness of the strained bulk active layer 3 is d, a confinement factor is xcex93, a photon energy is hv, a carrier lifetime is xcfx84, and a differential gain is a. A thickness d of the strained bulk active layer 3 is made small to make a confinement factor xcex93 small, whereby a large mode cross section (wxc3x97d/xcex93) is provided while, in addition, the effect of a higher carrier density reducing a carrier lifetime xcfx84 is provided, so as to provide a larger output saturation power.
On the other hand, the strained bulk active layer 3 is thinned and has a sectional configuration of higher flatness, whereby an optical confinement ratio between TE and TM polarization lights is high, and a required strain amount is larger. However, when the strained bulk active layer 3 has a thickness of 20 nmxcx9c100 nm, a strain amount is set to be xe2x88x920.09%xcx9cxe2x88x920.60%, whereby polarization independence can be retained.
When a thick strained bulk active layer 3 is sandwiched by thick separate confinement heterostructure layers 2, the optical confinement ratio between TE and TM polarization lights become smaller, whereby the strain amount required for polarization independence become smaller. In a case that the thickness of the strained bulk active layer 3 is 100 nm, in consideration of directing conditions of fundamental mode in a case that a stripe width is above 1 xcexcm, it is possible that the strained bulk active layer 3 is sandwiched by the separate confinement heterostructure layers 2 of a 300 nm-thick and of a 1.2 xcexcm composition (xcexg=1.2 xcexcm). In this case, the strain amount of the strained bulk active layer 3 required for the polarization independence is xe2x88x920.09%.
When a thin strained bulk active layer 3 is not sandwiched by separate confinement heterostructure layers 2, the optical confinement ratio between TE and TM polarization lights increases, whereby the strain amount required for polarization independence increases. In a case that the thickness of the strained bulk active layer 3 is 20 nm, the strain amount of the strained bulk active layer 3 required for the polarization independence is xe2x88x920.60% when the strained bulk active layer 3 is not sandwiched by the separate confinement heterostructure layers 2.
As will be described later, an upper limit of the thickness of the strained bulk active layer 3 is set to be about 90 nm or about 80 nm, whereby better output saturation power can be provided. A lower limit of the strain amount in the case that a thickness of the strained active layer 3 is 90 nm is xe2x88x920.10%, and in a case that a thickness of the strained active layer 3 is 80 nm, a lower limit of the strain amount is xe2x88x920.11%.
One measure of a lower limit of the strained bulk active layer thickness is considered to be about 20 nm, because when the strained bulk active layer 3 is thinned, and the quantum effect is conspicuous, a material gain for TE polarization light is large.
As will be described later, a lower limit of the thickness of the strained bulk active layer 3 is set to be about 25 nm or about 30 nm, the quantum effect can be effectively depressed. A upper limit of the strain amount in the case that a thickness of the strained active layer 3 is 25 nm is xe2x88x920.45%, and in a case that a thickness of the strained active layer 3 is 30 nm, a upper limit of the strain amount is xe2x88x920.44%.
The present invention is also characterized in that, in the above-described polarization independent-type semiconductor optical amplifier, the strained bulk active layer is sandwiched by separate confinement heterostructure layers 2 disposed in a direction of thickness of the strained bulk active layer 3 and in contact with the strained bulk layer 3.
Thus, the strained bulk active layer 3 is sandwiched by the separate confinement heterostructure (SCH) layers 2, whereby a smaller optical confinement ratio between TE and TM polarization lights and a larger mode cross section (wxc3x97d/xcex93) can be provided.
In this case, an optical axial direction of the strained bulk active layer 3 is inclined by 7-10xc2x0 to a normal of the light exit end face 8, whereby light resonance due to reflection between the light incident end face 7 and the light exit end face 8 can be depressed. The semiconductor optical amplifier can be free generation of ripples in outputs.
In this case, it is preferable that a width of the strained bulk active layer 3 is tapered to gradually reduce the width at a ratio of above 1/1000 per a unit length toward the light exit end face 8 from the center of the device, whereby optical coupling efficiency with an optical system, such as optical fibers, etc. is improved.
When a 1.0 xcexcm-width is reduced to a 0.6 xcexcm-width over a 400 xcexcm-length, the ratio is 1/1000 ((1.0 xcexcm-0.6 xcexcm)/400 xcexcm).
Otherwise, it is possible that a thickness of the strained bulk active layer is tapered gradually toward the light exit end face 8 from the center of the device so that a thickness of the strained bulk active layer 3 at the end on the light exit end face 8 is below xc2xd of a thickness thereof at the center of the device. Higher optical coupling with an optical system, such as optical fibers can be provided.
It is preferable that the so-called window structure, in which the strained bulk active layer 3 is absent, but the clad layer is present, is provided over 20-50 xcexcm from the light exit end face 8 on the light exit end face 8. Reflection of signal light 9 on the light exit end face 8 can be prevented, whereby generation of ripples in amplified signal light 10 can be prevented without failure.